Pneumatic tire having circumferential groove

ABSTRACT

A pneumatic tire, which comprises a tread portion provided with a circumferential groove having a pair of side walls, in the axial direction of the tire the circumferential groove is disposed within the ground contacting width of the tread under a standard loaded state, the standard loaded state is such that the tire is mounted on its regular rim and inflated to 70% of its maximum air pressure and then loaded with 88% of its maximum load, the axial width of the circumferential groove is in the range of from 0.2 to 0.35 times the ground contacting width, and at least one of the side walls has a nonrectilinear configuration having a plurality of axially deviated parts, whereby the conditions of the resonance of the air in the circumferential groove are continuously changed during running to decrease air resonance noise thereby effectively reducing the tire noise.

This application is a continuation of application Ser. No. 08/261,092filed Jun. 16, 1994, now abandoned, which is a divisional application ofSer. No. 08/148,913 filed on Nov. 5, 1993, now abandoned.

FIELD OF THE INVENTION

The present invention relates to a pneumatic tire in which tire noise isreduced without deteriorating wet performance.

BACKGROUND OF THE INVENTION

Recently, as the noise generated by the car mechanism and body becomesreduced, the percentage of the noise generated by the tire increases.Therefore, there is an increasing demand for a low noise tire.

For reducing the tire noise, it is especially effective to reduce arelatively high frequency noise to which the human ears are sensitive,that is, noise whose frequency range is around 1 kHz.

Air resonance noise is such a high frequency tire noise. Air resonancenoise is explained as follows.

In the ground contacting patch of the tire, an air tube is formed by theroad surface and a continuous circumferential groove which is providedin the tire tread portion. During running, the air tube is excited bythe air flow, a tread deformation or movement, roughness of the roadsurface and the like, and a noisy sound having a specific wave lengthwhich corresponds to two times the length of the air tube is generated.Most tire noise whose frequency range is about 800 to 1.2 kHz generatedby the tire having a circumferential groove is such air resonance noise.The frequency of the air resonance is almost constant independent ofrunning speed.

Since it has been known that air resonance can be prevented bydecreasing groove volume, decreasing groove volume and/or the number ofcircumferential grooves has been employed to reduce air resonance noise.As a result, wet performance such as aquaplaning, wet grip and the likeis inevitably decreased.

It is therefore, an object of the present invention to provide apneumatic tire in which, by using a very wide circumferential groove andcontinuously changing the conditions of the air resonance duringrunning, the air resonance noise is effectively reduced to decrease theoverall noise level without sacrificing wet performance.

The foregoing specific objects and advantages of the invention areillustrative of those which can be achieved by the present invention andare not intended to be exhaustive or limiting of the possible advantageswhich can be realized. Thus, these and other objects and advantages ofthe invention will be apparent from the description herein or can belearned from practicing the invention, both as embodied herein or asmodified in view of any variations which may be apparent to thoseskilled in the art. Accordingly, the present invention resides in thenovel parts, constructions, arrangements, combinations and improvementsherein shown and described.

SUMMARY OF THE INVENTION

According to one aspect of the present invention a pneumatic tirecomprises a tread portion provided with a circumferential groove havinga pair of side walls, in the axial direction of the tire thecircumferential groove is disposed within the ground contacting width ofthe tread under a standard loaded state, the standard loaded state issuch that the tire is mounted on its regular rim and inflated to 70% ofits maximum air pressure and then loaded with 88% of its maximum load,the axial width of the circumferential groove is in the range of frombetween 0.2 to 0.35 times the ground contacting width, and at least oneof the side walls has a nonrectilinear configuration having a pluralityof axially deviated parts.

It will be appreciated by those skilled in the art that the foregoingbrief description and the following detailed description are exemplaryand explanatory of the invention, but are not intended to be restrictivethereof or limiting of the advantages which can be achieved by theinvention. Thus the accompanying drawings, referred to herein andconstituting a part hereof, illustrate preferred embodiments of theinvention and, together with the detailed description, serve to explainthe principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of this inventionwill be apparent from the following detailed description, especiallywhen taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a cross sectional view of a pneumatic tire according to thepresent invention;

FIG. 2 is a cross sectional view of the circumferential groove thereof;

FIG. 3 is a plan view showing an example of the configuration of thecircumferential groove;

FIG. 4 is a plan view showing an example of a tread pattern in which thecircumferential groove of FIG. 3 is used;

FIG. 5 is a plan view showing another example of the tread pattern inwhich the circumferential groove of FIG. 3 is used;

FIG. 6 is a plan view showing another example of the configuration ofthe circumferential groove;

FIG. 7 is a plan view showing still another example of the configurationof the circumferential groove and an example of the tread pattern;

FIG. 8 is a plan view showing still more another example of theconfiguration of the circumferential groove;

FIG. 9 is a plan view showing another example of the location of thecircumferential groove of FIG. 7;

FIG. 10 is a plan view showing another example of the configuration ofthe circumferential groove and an example of the tread pattern;

FIG. 11 is a plan view showing another example of the configuration ofthe circumferential groove and an example of the tread pattern;

FIG. 12 is a plan view showing another example of the location of thecircumferential groove of FIG. 11; and

FIG. 13 is a plan view showing another example of the configuration ofthe circumferential groove.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention can be applied to various types of tires, butsuitably applied to a low aspect ratio passenger radial tire.

FIG. 1 shows an example of the tire structure for a low aspect ratiopassenger radial tire, in which the tire 1 comprises a tread portion T,a pair of axially spaced bead portions B, a pair of sidewall portions Sextending between the tread edges and the bead portions, a pair of beadcores 2 each disposed in each of the bead portions, a radial carcass 3extending between the bead portions and turned up around the bead cores2 from axially inside to outside of the tire, a belt 4 disposed radiallyoutside the carcass and inside a rubber tread, and a bead apex 6disposed radially outside each bead core 2 and between the main portionand each turned up portion of the carcass.

The carcass 3 comprises at least one ply of radially arranged organicfiber cords, e.g. nylon, rayon, polyester and the like.

The belt 4 comprises at least two cross plies of high modulus cords suchas steel, aromatic polyamide and the like, laid at a relatively smallangle of from 15 to 30 degrees with respect to the tire circumferentialdirection.

The aspect ratio (height/width ratio) of the tire 1 is in the range offrom about 0.4 to 0.6.

The tread portion T is provided with a convex profile. The tread profilemay be of a single radius curvature, but in general, the tread profilehas a multi-radius curvature such that a major central portion thereofhas a single radius R1 larger than the radius R2 of the shoulderportions, and the central portion and shoulder portions are continuedwithout forming an angle.

The tread portion T is provided with a wide circumferential groove 7extending continuously in the tire circumferential direction of the tireand a plurality of axial grooves.

In the present invention, a state of the tire which is mounted on itsregular rim R and inflated to 70% of its maximum air pressure and thenloaded with 88% of its maximum load, is referred to as the standardloaded state. In this regard, the maximum air pressure and the maximumload are those officially specified for the tire by for example JATMA(Japan), TRA (USA), ETRTO (Europe) and the like, and the regular rim isa rim officially specified for the tire by the same associations for thepurpose of measuring the tire dimensions.

According to the present invention, the circumferential groove 7 has avery large width as compared with conventional widths.

The width ratio Wg/Sw of the axial width Wg of the circumferentialgroove 7 to the ground contacting width Sw is in the range of from 0.20to 0.35. The ground contacting width Sw is the maximum axial width ofthe ground contacting area S under the above-mentioned standard loadedstate.

In the case of passenger radial tires, for which the nominal sectionwidth in a metric tire size designation is in the range of from 155 to225, the width Wg of the circumferential groove 7 is in the range offrom 30 to 70 mm, preferably, 35 to 50 mm.

As shown in FIG. 2, the circumferential groove 7 has a flat bottom 7Cand a pair of side walls 7A and 7B. The radially inner end of each ofthe side walls 7A and 7B is inclined to the center of the groove, andthe inclination angle (alpha) thereof is 0 to 30 degrees, preferably 5to 20 degrees, with respect to a radial line X at a right angle to thetire axis.

The intersecting part between the groove bottom 7C and the side walls 7Aand 7B is rounded by a small radius (r) of curvature. The radius (r) isin the range of from 0.1 to 0.5 times the groove depth D. However, theintersecting part between the tread face and the side walls 7A and 7B isedged thereby improving the road grip.

The groove depth D of the circumferential groove 7 is set in the rangeof from 10 to 20% of the width Wg.

In the above-mentioned passenger tires, the depth D is preferably set inthe range of from 5 to 10 mm. If the depth is less than 5 mm, thedrainage performance is low. If the depth is more than 10 mm, it becomesnecessary to increase the tread rubber thickness, which results in anincrease in the tire weight, which is especially undesirable forpassenger tires.

Further, the circumferential groove 7 is disposed within a tread centralregion which comprises about 70% of the width of the ground contactingwidth Sw and is centered on the tire equator CO. If the circumferentialgroove 7 protrudes from this region, the vibration mode of the belt edgeexcites the air resonance of the circumferential groove 7 increasingresonance noise.

As explained above, by setting the groove width relatively large, in thecircumferential center of the ground contacting part of the tread, thebottom of the circumferential groove 7 becomes convex as shown byexaggeration in FIG. 2 by the broken line. Further, the tread rubber iscompressed. Accordingly, the cross sectional area of the middle groovepart becomes smaller than the forward and backward groove part. As aresult, the resonance produced by the groove is altered in order tothereby reduce the resonance noise.

If the width ratio Wg/Sw is less than 0.20, an effective deformation ofthe groove or air tube cannot be obtained, and therefore the airresonance noise is not reduced, and further an aquaplaning phenomenon isliable to occur. If the width ratio Wg/Sw exceeds 0.35, the noisereduction effect does not increase any more, but the cornering force isdecreased and the dry grip performance and steering stability aredeteriorated.

In the present invention, in order to eliminate the source of resonancenoise, a circumferentially continuous groove which has a conventionalwidth ranging from about 10 to 20 mm is not provided in the treadportion. A very narrow groove and a zero-width cut can be provided, butpreferably, only one circumferential groove 7 is provided.

By the circumferential groove 7, the tread portion T is divided into twolateral tread parts 9.

Each lateral tread part 9 is provided with a plurality of axial grooves10 so as to set a sea/land ratio Ss/Sr in the range of from 0.30 to0.50. Here, the sea/land ratio Ss/Sr is a ratio of the sea area Ss tothe land area Sr, both in the above-mentioned ground contacting area Sunder the standard loaded state. The sea area Ss is the total area ofthe grooved part, and the land area Sr is the total area of theremaining part or the actual ground contacting area.

If the sea/land ratio Ss/Sr is less than 0.30, the wet grip performanceespecially during cornering is deteriorated. If the sea/land ratio Ss/Sris more than 0.50, the cornering force is decreased to deteriorate thesteering stability.

As explained above, air resonance noise is effectively reduced bysetting the width of the circumferential groove over the conventionalwidth range.

However, it was discovered that a further noise reduction can beachieved by continuously changing the resonance mode during running. Toachieve this, at least one of the side walls 7A and 7B is formed in anonrectilinear configuration, e.g. zigzag, wavy, stepped and the like soas to form portions having an axial deviation along the tirecircumferential direction.

FIG. 3 shows an example of the configuration of the circumferentialgroove 7. In this example, both the side walls 7A and 7B extend in azigzag configuration. The zigzag of each side wall 7A, 7B consists oftwo different cycles having different amplitudes and arrangedalternately in the tire circumferential direction. However, the zigzagpattern can be made up of more than two different cycles or only onecycle. Each zigzag cycle is defined by straight circumferential segments(7s1, 7s2, 7s3, 7s4) extending parallel with the tire equator CO, andstarlight inclined segments (7i1, 7i2, 7i3, 7i4) extending between theadjacent circumferential segments. The straight circumferential segments(7s1, 7s2) have deviations D1 from the center line C, which have thesame smaller amplitude but in the opposite directions. The straightcircumferential segments (7s3, 7s4) have deviations D2 from the centerline C, which have the same larger amplitude but in the oppositedirections. All the circumferential segments 7s1 to 7s4 and the inclinedsegments 7i1 to 7i4 have substantially the same circumferential lengthL. However, the circumferential lengths of the segments can be variedfrom one another. Further, the circumferential lengths of the zigzagcycles can be varied.

The side walls 7A and 7B are in parallel with each other. However, thezigzag configuration of one of the side walls 7A and 7B can be shiftedfrom that of the other in the tire circumferential direction.

Preferably, the peak-to-peak amplitude DM of deviation is smaller than30%, more preferably smaller than 20% of the above-mentioned groundcontacting width Sw. The peak-to-peak amplitude is the axial distancebetween the circumferentially adjacent peaks of the opposite deviations.If the peak-to-peak amplitude DM exceeds 30%, the drainage of thecircumferential groove becomes decreased, and the resultant protrudingpart of the tread rubber into the groove is liable to wear earlier, anduneven wear starts therefrom.

If the axial width of the circumferential groove 7 is changed in thetire circumferential direction due to the deviation, the mean axialwidth is used for the above-mentioned axial width Wg.

In practice, the circumferential groove 7 of FIG. 3 is used incombination with the axial grooves as shown in FIGS. 4 and 5.

In an example of the tread pattern shown on FIG. 4, the circumferentialgroove 7 is centered on the tire equator CO, and a plurality of axialgrooves 10 (10a, 10b) are disposed on each side of the circumferentialgroove 7. In this example, the axial lengths K1 and K2 of the axialgrooves 10a and 10b respectively, are varied in proportion to the axialwidth of the lateral tread part 9 measured from each tread edge to thecircumferential groove 7, whereby the variation of the tread rigidity inthe tire circumferential direction is decreased and the runningstability and uneven wear can be improved.

In another example of the tread pattern shown in FIG. 5, thecircumferential groove 7 is disposed asymmetrically about the tireequator CO. The center line CL of the circumferential groove 7 is setaside from the tire equator CO by a distance (d). Preferably, thedistance (d) is less than about 20% of the ground contacting width Sw.

The tire having this asymmetrical tread pattern is used so that thetread edge 13a being farther from the circumferential groove 7 islocated on the outside of a car, and the nearer tread edge 13b on theinside of the car, whereby the outside tread part, which is subjected toa larger load than the inside tread part during cornering, is increasedin rigidity to improve cornering performance. In this example, the axiallengths of the axial grooves 10 disposed on each side of thecircumferential groove 7 are not varied.

FIG. 6 shows another example of the configuration of the circumferentialgroove 7, which is a modification of the circumferential groove 7 ofFIG. 3. In this example, the side wall 7B is modified so as to extendstraight in parallel with the tire equator CO, but the side wall 7Aremains unmodified. The circumferential groove 7 of this example iscentered on the tire equator CO, but it can be set aside the tireequator CO by a distance (d).

FIGS. 7 and 8 show other examples of the configuration of thecircumferential groove 7, in which the side walls 7A and 7B extendzigzag or wavily in parallel with each other. In the example of FIG. 7,each of the side walls 7A and 7B of the circumferential groove 7 extendszigzag and consists of alternate straight segments 7a and straightsegments 7b which are inclined in opposite directions. In the example ofFIG. 8, each of the side walls 7A and 7B of the circumferential groove 7extends wavily. In these examples, the amplitude of the deviation is setto satisfy the following relationship: ##EQU1## wherein A is theamplitude which is an axial distance between the circumferentiallyadjacent peak points (a) and (b) of the center line CL of thecircumferential groove 7,

P is the pitch which is a circumferential distance between theabove-mentioned peak points (a) and (b), and

W is the groove width which is the width measured at a right angle tothe center line CL of the circumferential groove 7.

If the amplitude (A) is outside the above-mentioned range, thesuppression of the air resonance and the dispersion of the noise to awide frequency range become insufficient.

Preferably, the amplitude (A) is not more than 25% of the groundcontacting width SW. If the amplitude (A) is more than 25%, the angle ofthe circumferential groove to the tire circumferential direction becomestoo large, and the rigidity of the lateral tread part is decreasedthereby deteriorating the steering stability, and resulting in unevenwear resistance.

The pitch P is in the range of from 0.8 to 3 times the ground contactinglength SL, which is the circumferential length of the ground contactingarea under the above-mentioned standard loaded state. If the pitch P isless than 0.8 times the ground contacting length SL, the drainageperformance is decreased. If the pitch P is more than 3 times SL, theair resonance noise can not be improved.

In FIG. 7, the circumferential groove 7 is centered on the tire equatorCO, and a plurality of axial grooves 10 are disposed on each sidethereof. The axial lengths of the axial grooves 10 are varied inproportion to the axial width of the lateral tread part 9.

FIG. 9 shows another example of a location for the zigzagcircumferential groove 7 of FIG. 7. In this example, the groove 7 is setaside from the tire equator CO. Similar to the FIG. 5 example, the tirehaving this asymmetrical tread pattern is used such that the tread edge13a being farther from the circumferential groove 7 is located on theoutside of a car, and the nearer tread edge 13b on the inside of thecar.

In FIGS. 7 and 9, the zigzag pitch P is constant in the circumferentialdirection, but it can be varied.

FIG. 10 shows another example of a configuration for the circumferentialgroove 7, which is a modification of the above-mentioned zigzagcircumferential groove 7. In this example, the zigzag consists ofalternate long straight segments 7a and short straight segments 7b whichare inclined in opposite directions. In each pitch, the above-explainedrelationship is satisfied. That is, in the long pitch P1 defined by thelong segment 7a, ##EQU2## In the short pitch P2 defined by the shortsegment 7b, ##EQU3## By using a plurality of different pitches, thenoise dispersion effect can be enhanced.

FIG. 11 shows another example of the circumferential groove 7. In thisexample, the circumferential groove 7 extends generally straight in thetire circumferential direction, but the axial width thereof isperiodically decreased along the length thereof to form a plurality ofnarrow width portions 16.

The axial width Wm of the circumferential groove 7 at each of the narrowwidth portions 16 is in the range of from more than 0 but not more than0.8 times the maximum axial width WM of the circumferential groove 7.Preferably, the width Wm is in the range of from 0.1 to 0.8 times, morepreferably in the range of from 0.2 to 0.6 times the maximum width WM.The maximum width WM is set in the range of from 30 to 70 mm in case ofpassenger tires as explained above.

The circumferential length P3 of the narrow width portion 16 is verysmall in comparison to the circumferential length P4 of the remainingwider portion 17.

Therefore, the pitch of the one repeating cycle, which is the total ofthe length P3, the length P4 and the circumferential length of thetransitional part between the wide portion 17 and the narrow portion 16,is set to be smaller than the ground contacting length so that duringrunning at least one narrow width portion 16 always exists within theground contacting area S.

In this example, in order to form the narrow width portions 16, the sidewalls 7A and 7B are provided at the same circumferential positions withinwardly protruding parts 16 (deviations). As a result, thecircumferential groove 7 is symmetrically disposed about the center linethereof.

The circumferential groove 7 is centered on the tire equator CO, but itcan be set aside from the tire equator CO as shown in FIG. 12.

FIG. 13 shows another example of the circumferential groove 7, which isa modification of the FIG. 11 groove. In this example, the protrudingparts 16 of the side wall 7A and those of the other side wall 7B arestaggered in the circumferential direction. As a result, thecircumferential groove 7 is asymmetrical about the center line CLthereof. The widths Wm and WM also satisfy the above-explainedconditions. The present invention provides a pneumatic tire comprising:

a tread portion with a ground contacting area, the ground contactingarea having a sea/land ratio in the range of 0.30 to 0.50,

said tread portion provided with a single circumferential groove havinga pair of sidewalls defining two lateral portions on each side of thecircumferential groove,

said circumferential groove being disposed within the ground contactingwidth of the tread portion in the axial direction of the tire,

the ground contacting width being the maximum axial width of the groundcontacting area of the tread portion under a standard loaded state wheresaid standard loaded state is such that the tire is mounted on a regularrim, inflated to 70% of its maximum air pressure and then loaded with88% of its maximum load, characterized in that

the axial width of said circumferential groove is in the range of from0.2 to 0.35 times the ground contacting width,

said circumferential groove having edges and being the only groove whichextends continuously in the tire circumferential direction, and

each said sidewalls has a plurality of inwardly protruding parts so that

the width of the circumferential groove is periodically decreased in thelongitudinal direction thereof so as to form narrow width portionshaving a first positive circumferential length, wide width portions witha second positive circumferential length and intermediate portionsrespectively connecting each of the wide and narrow width portions,along said longitudinal direction,

the width at each said narrow width portion is more than 0 but not morethan 0.8 times the maximum width of the circumferential groove, and

the maximum width of the circumferential groove is in the range of from30 to 70 mm, and

the protruding parts provided on one of the sidewalls arecircumferentially shifted from the protruding parts provided on theother sidewall, whereby the configuration of the circumferential grooveis not symmetrical about the center line thereof wherein the edges ofthe circumferential groove in each of said narrow width portions andsaid wide width portions are substantially parallel to thecircumferential direction, each narrow width portion is defined by onlyone of said protruding parts, and the second positive circumferentiallength of each of the wide width portions is greater than the firstpositive circumferential length of each of the narrow width portions.

In FIGS. 4, 5, 7, 10, and 11, the axially outer ends of the axialgrooves 10 are extended to the tread edges. The axially inner endsthereof are not extended to the circumferential groove 7, and they areterminated in the respective lateral tread portions 9, whereby therigidity of the lateral tread parts 9 is increased to improve thesteering stability and uneven wear resistance.

The width of the axial grooves 10 is in the range of from 5 to 15% ofthe axial width Wg of the circumferential groove 7. Preferably, thewidth of the axial groove is gradually increased from the inner end tothe outer end thereof. Also preferably, the number of the axial groovesis set such that during running two to four axial grooves always existwithin the ground contacting area S on each side of the circumferentialgroove 7. If the number is less than two, the drainage performancebecomes insufficient. If more than four, the tread rigidity isdecreased, and the steering stability is lost.

Each of the axial grooves 10 is curved such that the angle of the groovecenter line with respect to the tire circumferential direction isgradually increased from the inner end toward the outer end thereof. Ineach lateral tread part 9, the axial grooves 10 are inclined in the samedirection.

In FIGS. 4 and 5, the axial grooves 10 in one of the lateral tread parts9 are inclined in the opposite direction to the axial grooves 10 in theother lateral tread part 9. The angle of each of the axial grooves isgradually increased from the axially inner end thereof to the axial edgeline Y of the ground contacting area, and then the angle is graduallydecreased to the axially outer end thereof, inclining to the reversedirection. Accordingly, the maximum angle of 90 degrees lies at theaxial edge line Y. The minimum angle at the axially inner end is in therange of from about 50 to 70 degrees.

In FIGS. 7, 10 and 11, the axial grooves 10 in one of the lateral treadparts 9 are inclined in the same direction as the axial grooves 10 inthe other lateral tread part 9. The angle of each of the axial groovesis gradually increased from the axially inner end to the outer endthereof. The maximum angle at the axially outer end is about 50 to 70degrees. The minimum angle at the axially inner end is about 40 to 50degrees. The angle at the middle point therebetween is less than 60degrees.

In addition to the above-mentioned axial grooves 10, in order to improvewet grip and/or adjust the tread rigidity, sipes can be provided in thetread portion, wherein the sipe is a cut whose width is in the range offrom 0.2 to 0.7 mm. Various sipes, e.g. a curved sipe, a straight sipe,a sipe connected with the circumferential groove 7, a sipe disconnectedwith the circumferential groove 7, etc. may be used.

In order to prevent a possible excessive deformation of the bottom ofthe circumferential groove 7, the circumferential groove 7 can beprovided in the bottom with protrusions spaced apart from each other inthe longitudinal direction of the groove. The height of such protrusionsis preferably in the range of from 50 to 15% of the depth D of thecircumferential groove 7.

Although illustrative preferred embodiments have thus been describedherein in detail, it should be noted and will be appreciated by thoseskilled in the art that numerous variations may be made within the scopeof this invention without departing from the principle of the inventionand without sacrificing its chief advantages. The terms and expressionshave been used as terms of description and not terms of limitation.There is no intention to use the terms or expressions to exclude anyequivalents of features shown and described or portions thereof and theinvention should be defined in accordance with the claims which follow.

We claim:
 1. A pneumatic tire comprising:a tread portion with a groundcontacting area, the ground contacting area having a sea/land ratio inthe range of 0.30 to 0.50, said tread portion provided with a singlecircumferential groove having a pair of sidewalls defining two lateralportions on each side of the circumferential groove, saidcircumferential groove being disposed within the ground contacting widthof the tread portion in the axial direction of the tire, the groundcontacting width being the maximum axial width of the ground contactingarea of the tread portion under a standard loaded state where saidstandard loaded state is such that the tire is mounted on a regular rim,inflated to 70% of its maximum air pressure and then loaded with 88% ofits maximum load, characterized in that the axial width of saidcircumferential groove is in the range of from 0.2 to 0.35 times theground contacting width, said circumferential groove having edges andbeing the only groove which extends continuously in the tirecircumferential direction, and each said sidewalls has a plurality ofinwardly protruding parts so that the width of the circumferentialgroove is periodically decreased in the longitudinal direction thereofso as to form narrow width portions having a first positivecircumferential length, wide width portions with a second positivecircumferential length and intermediate portions respectively connectingeach of the wide and narrow width portions, along said longitudinaldirection, the width at each said narrow width portion is more than 0but not more than 0.8 times the maximum width of the circumferentialgroove, and the maximum width of the circumferential groove is in therange of from 30 to 70 mm, and the protruding parts provided on one ofthe sidewalls are circumferentially shifted from the protruding partsprovided on the other sidewall, whereby the configuration of thecircumferential groove is not symmetrical about the center line thereofwherein the edges of the circumferential groove in each of said narrowwidth portions and said wide width portions are substantially parallelto the circumferential direction, each narrow width portion is definedby only one of said protruding parts, and the second positivecircumferential length of each of the wide width portions is greaterthan the first positive circumferential length of each of the narrowwidth portions.
 2. The pneumatic tire according to claim 1, wherein eachsaid sidewall has a peak-to-peak amplitude of smaller than 30% of theground contacting width.
 3. The pneumatic tire according to claim 1,wherein said circumferential groove is disposed within a tread centralregion having a width of 70% of the ground contacting width.
 4. Thepneumatic tire according to claim 1, wherein the circumferential grooveis centered on the tire equator.
 5. The pneumatic tire according toclaim 1, wherein said width at each of the narrow width portions is inthe range of from 0.1 to 0.8 times the maximum width of thecircumferential groove.
 6. The pneumatic tire according to claim 1,wherein said width at each of the narrow width portions is in the rangeof from 0.2 to 0.6 times the maximum width of the circumferentialgroove.
 7. The pneumatic tire according to claim 1, wherein saidplurality of narrow width portions, wide width portions and intermediatewidth portions occur along the circumferential groove such that at leastone narrow width portion always exists in the ground contracting areafor preventing the formation of air resonance resulting from an air tubeoccurring between the ground and the tread portion in a section of thecircumferential groove within the ground contacting area.
 8. Thepneumatic tire according to claim 1, wherein said tread portion isprovided with no groove being connected to said circumferential groove.9. The pneumatic tire according to claim 8, whereinsaid lateral portionsare provided with a plurality of axial grooves extending to tread edges,and the number of the axial grooves is set such that on each side of thecircumferential groove during running two to four axial grooves alwaysexist within the ground contacting area.